The present subject matter relates to a method of fabricating an integrated circuit device. More particularly, the present subject matter presents a method of stripping photoresist after high dose and high energy ion implantation during the fabrication process. However, the method is also useful for stripping photoresist after other processes that alter the composition of the photoresist.
Photoresist is a key component of the process by which integrated circuits in semiconductor devices are fabricated. A photoresist solution is typically coated on a substrate, baked to form a film, patternwise exposed, and developed in aqueous base such as 0.26 Nt-butyl ammonium hydroxide in water to form a pattern in the film. Film thickness is usually in the range of 0.2 to 2 microns but may be thinner or thicker depending on the application. The photoresist can be positive tone in which the exposed area becomes soluble in the developer while the unexposed portion remains insoluble or negative tone where the exposed portion becomes crosslinked or is otherwise made to be insoluble in the developer while the unexposed portion dissolves and is removed. In either case, the photoresist film typically comprises a polymer or resin, a photoactive compound, additives such as surfactants to improve film coating properties or performance, and a trace amount of solvent. For exposure wavelengths which are below 230 nm, one skilled in the art will appreciate that the polymer structure will be modified by removing aromatic groups in order to keep optical absorbance at a reasonable level, typically <1 per micron film thickness. Other types of radiation such as electron beam, ion beam or X-ray may be used to pattern the film.
An antireflective coating (ARC) may be applied on the substrate prior to coating the photoresist. The ARC prevents the imaging radiation from reflecting off the substrate and exposing the photoresist a second time and thereby controls the intensity of the radiation which exposes the photoresist. Once the photoresist film is imaged, the pattern may be transferred into an underlying layer by means of a selective dry etch process where the photoresist serves as a mask for the etching step. If an antireflective layer is present, one etch step is used to etch through the ARC and a second step is needed to transfer the pattern into the underlying substrate. The reactive ions in the dry (plasma) etch step can transform the photoresist film to render it less soluble insolvents or more difficult to remove by an ashing method.
The photoresist must be removed or stripped before subsequent processing steps to avoid any contamination of the device which would cause reduced performance. Typically, the process of photoresist coating and imaging followed by dry etching, ion implantation or a deposition step, and then stripping the photoresist is repeated several times in order to build several layers required for the device.
The photoresist is selected from a group of imaging materials consisting of single layer, bilayer, multilayer, or surface imaging materials. A bilayer or multilayer imaging system typically has a photoresist in the top layer which contains silicon and a lower layer or layers that are not photosensitive. After the top layer is exposed and developed to form a pattern, it serves as an excellent etch mask for image transfer using oxygen plasma into the lower layer or layers. In the presence of oxygen plasma, the silicon materials form silicon dioxide which dramatically slows down the etch loss of the top layer. The lower layer is usually thicker than the top photoresist layer in a bilayer system and serves as an etch mask for image transfer into an underlying substrate.
Silicon containing resists are often difficult to strip because the top layer is covered with a layer of silicon dioxide after the etch transfer step and silicon dioxide is only removed very slowly in oxygen plasma. An alternative to traditional photoresist processing in which the film is developed in aqueous base solution is top surface imaging (TSI). In a TSI process, the photoresist film is usually very opaque to the imaging radiation such that the incident radiation penetrates only part way into the film. The film is then treated with a vapor containing a silicon compound at various temperatures and pressures to selectively form Si—O bonds in either the exposed or unexposed portions of the film. Rather than develop the pattern in aqueous base solution, the film is dry etched in plasma to selectively remove film in regions without any silicon content.
A photoresist pattern can also function as a mask for ion implant steps in which ions of phosphorus, arsenic, or boron are implanted at high energy into an underlying layer. Again, the high energy process can transform the resist film, especially the top portion of the film, such that it becomes difficult to strip. Both wet and dry stripping methods are known. Strippers which involve organic solvents often contain amines that can contaminate the air and adversely affect sensitive photo imaging processes in the fab. They are also costly because of disposal requirements.
Another wet stripper is Caro's acid which consists of H2SO4 and H2O2. This solution is a strong oxidant and reacts with photoresist materials composed of the elements C,H, N, S, and O to form the corresponding oxides such as CO2 and H2O. However, when the surface of the photoresist film becomes altered during high energy ion bombardment, Caro's acid is no longer an efficient oxidant.
Dry etching in oxygen plasma also reacts with photoresist materials to form oxides such as CO2, H2O SO2 and NO2. While dry stripping is preferred, a better process is needed in terms of higher throughput. Ion implantation or other processing steps may harden the photoresist film to the extent that several cycles through an etching chamber or two or more different etching steps are required to completely remove the photoresist film. Ion implantation may dehydrate the resin and make the photoresist surface more compact. Ion implantation also tends to make the oxidation reaction at the surface of the film more difficult. A combination of CF4 and O2 in the etching gas may be used to more easily remove the photoresist. However, this technique is likely to have a detrimental effect on the substrate that reduces device performance. Therefore, an improved stripping method is needed that does not damage the device and does not require a lengthy process time including two or more dry etch steps.
A primary objective of semiconductor manufacturing is to minimize costs such that the product is competitive in the marketplace. Improved processes which decrease throughput time are desirable because they reduce cost. These improvements may consist of fewer steps in a process or a shorter amount of time in a processing tool.
U.S. Pat. No. 6,043,004 describes a two step oxygen etch method to strip photoresist that has been impacted by a high dose ion implant process. However, a critical step is to gradually increase the temperature of the wafer from 150° C. to 200° C. which requires a lengthy process time. U.S. Pat. No. 5,811,358 describes a stripping method consisting of three dry etch steps at different temperatures followed by two wet steps. This is a multi-step process that could impact throughput.
U.S. Pat. No. 5,882,489 mentions the use of an ashing step followed by an argon sputtering step to cleanly remove photoresist. This method would require two different etch chambers and thus have an increased equipment cost over a method with only one etching step.
U.S. Pat. Nos. 6,006,764 and 5,731,243 describe methods to remove photoresist from bonding pads.
The present subject matter obviates the deficiencies in the prior art and provides a method for stripping photoresist after the photoresist film has been subjected to a high dose and high energy implant process. The present subject matter also provides a method for stripping photoresist in applications where the photoresist film has a hardened skin or a high molecular weight polymer resulting from a previous processing step.
In one embodiment, the present subject matter presents a method for stripping photoresist during the fabrication of integrated circuits comprising: treating the photoresist film on a wafer by soaking in de-ionized (DI) water, dry etching the wafer with an oxygen plasma to remove the photoresist, immersing the wafer in Caro's acid to remove any residues, and rinsing and drying the wafer. The de-ionized water treatment may be combined with the dry etching in an integrated step or the two steps may be performed separately. In another embodiment, the present subject matter is a method for stripping photoresist during the fabrication of integrated circuits comprising: treating the photoresist film on a wafer by soaking in de-ionized (01) water, immersing the wafer in Caro's acid to remove the photoresist, and rinsing and drying the wafer.
In another embodiment, the present subject matter presents a method for high dosage and high energy ion implantation during the fabrication of integrated circuits comprising: coating a photoresist solution on a substrate and baking to form a film, patternwise exposing the photoresist film to radiation, thereby forming a pattern of exposed regions in the film which is subsequently developed in aqueous base to uncover portions of the substrate, ion implanting with phosphorus, arsenic or boron ions at high energy through the gaps in the photoresist film into the underlying substrate, and stripping the photoresist wherein the film is soaked in DI water followed by treatment with Caro's acid and wherein the DI water soak may be combined with an oxygen etch step in a integrated process.
The present subject matter is particularly useful in that it provides a high throughput, low cost photoresist removal process because of a minimum number of steps and avoids the use of dry etching techniques such as a combination of CF4 and O2 gases that can affect the substrate and reduce the performance of the device.
These and other advantages of the disclosed subject matter will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.